For an ultrasonic measurement of the depth of a flaw opened on the surface of a solid mass, such as weld cracking in a welded portion, a fatigue crack in a member portion in which stresses concentrate, etc., various methods have recently been studied because of the necessity for the measured information, and the results from the studies have been reported. For example, (1) the measurement of the fatigue-crack depth by the end peak echo method is reported in the "Nondestructive Inspection" Vol. 31, No. 9, September, 1982, pp. 690-691, (2) a method for inspection for cracks using scattered ultrasonic waves is disclosed in the "Nondestructive Inspection" Vol. 29, No. 2, February, 1980, pp. 136-137, (3) the measurement of the height of incomplete penetration in a welded portion of steel plates is disclosed in the "Non-destructive Inspection" Vol. 34, No. 2, February, 1985, pp. 112-113, and (4) the accuracy of measurement of the notch depth based on time lapse of ultrasonic wave is described in the "Nondestructive Inspection" Vol. 29, No. 2, February, 1980, pp. 130-131.
In the measurement disclosed in the report (1), a spot-focusing type longitudinal wave angle probe is used to measure the relation between the beam path and various kinds of depth of slits formed axially in the inner wall of the bend of a pipe from the outer circumference of the pipe by the end peak echo method, thereby determining the crack depth from a calibration curve prepared based on the measured beam path. In the method for crack measurement using scattered ultrasonic waves in the above report (2), a probe is used which has arrayed therein an ultrasound transmitting transducer and a receiving transducer arrayed symmetrically with respect to a partition sheet. When ultrasonic waves are radiated from the transmitting transducer toward an object immersed in water, and a difference in time .DELTA.t of the reception of scattered waves by the receiving transducer between when the object has a flaw when the object has no flaw is measured. Utilizing the correlation between the time difference .DELTA.t and the flaw depth d, the flaw depth d is determined. In the method described in the above report (3), a two-transducer vertical-type probe is used and placed on the one-side butt joint of a plate finished smooth and having a thickness t (9 to 12 mm in the report). The depth of penetration d in the welded portion is directly read from the time base of the ultrasonic flaw detector and compared with the plate thickness, thereby measuring the height of incomplete penetration h= (t-d). In the method disclosed in the above report (4), an ultrasonic flaw detector permitting measurement of the time of ultrasound propagation with a high accuracy is used to determine the notch depth based on the propagation time measured by any of the end peak echo method, short-pulsed shear wave method or surface wave method. Among the methods disclosed in these reports, the method in the report (1) needs to detect the position of the peak echo from the end of the fatigue crack. The method in the report (2) necessitates the detection of the rise time of the waveform of the received scattered wave. Also, the method in the report (3) requires the detection of echoes from an incomplete penetration and positions of the echoes on the time base, which are displayed on the ultrasonic flaw detector. More particularly, the method in the report (2) is a method in which the measurement is done using a water bath in which the object is immersed and the probe must be placed just above the surface opening flaw in the object. Thus, this method can be applied to a limited range of objects. The method in the report (3) is done with the transducer placed directly on the object and is applicable to a limited range of objects as described with the method in the report (2). In the method disclosed in the report (4), the end peak echo method and short-pulsed shear wave method necessitate the detection of the positions of the peak echoes as in the method in the report (1), and the surface wave method needs the detection of the displayed positions of echoes on the CRT screen because the depth of the notch is to be determined from the propagation time of the surface wave. Thus, in all the methods disclosed in the reports (1) thru (4), it is necessary to detect the echoes from the surface opening flaw and the positions of appearance of echoes, and the accuracy of the measurement depends upon the accuracy of this detection.
Among the above-mentioned methods, the end peak echo method is generally regarded as the most popular one. A method of measuring the depth of a surface opening flaw using this method is described in the "Ultrasonic flaw-detecting test B" - Japan Nondestructive Inspection Association (1978), pp. 117 to 118. The method of measuring under consideration will be outlined with reference to FIGS. 11 and 12. In these Figures, the reference numeral 10 indicates an object under measurement in which a planar flaw 10a is intentionally formed which has a depth d and opens on the surface of the object. The reference numeral 10b indicates a searching surface of the object 10, 10c indicates the end of the flaw 10a and 10d indicates the bottom (opposite to the searching surface 10) of the object 10. The reference numeral 20 indicates an ordinary angle probe or spot-focusing angle probe (which will be referred to as "angle probe" hereinafter). The angle probe 20 is placed in contact with the searching surface 10b, and transmits ultrasonic waves while being scanned back and forth in the direction of arrow A or B so as to pick up the echoes from the end 10c of the flaw. The reference numerals 20a and 20b indicate the selected positions of the angle probe 20 when scanned back and forth in relation to the flaw. Assume here that the angle probe 20 is scanned in the direction of arrow B from the position 20a. The echo level from the flaw 10a is displayed as continuously changed so as to gradually be lower as the angle probe 20 is moved away from the position 20a, thereby resulting in an echo envelope 50. In this case, when the beam center 30 from the angle probe 20 is incident upon the end 10c of the flaw, some peak echo 60 is displayed in a position on the CRT screen corresponding to the beam path x and the position is indicated on the echo envelope 50. The end peak echo method is such that the depth d of flaw is determined geometrically as d=x.multidot.cos .theta. from the beam path x of the peak echo 60 from the end 10c of the flaw and the refraction angle .theta. of the ultrasonic beam from the angle probe 20. In the report (1), it is described that under the conditions that the angle .theta. formed between the incident direction of the ultrasonic wave and the plane of the flaw 10a is more than 10 deg., an ordinary angle probe of 45 deg. in refraction angle, a spot-focusing angle probe which can limit the acoustic waves or a split-type probe should preferably be used for more definite discrimination of the peak echo 60. It is further described in the report that if the depth d of the flaw is relatively large, the depth d can be estimated with an accuracy on the order of +/-2 mm. However, the report also reads that in case any other flaw exists at the end 10c of the flaw, the accuracy of measurement is lower.
FIG. 13 shows another example of measurement by the above-mentioned end peak echo method. In this example, the angle probe 20 is placed on the surface 11a of the object in which the flaw 11a is not existent. In an echo envelope obtained in this case, the echo from the surface opening of the flaw 11a (corner echo) shows the highest peak. However, in case the depth h of the flaw 11a is small, for example, 2 mm or so, the echo from the end 11c of the flaw 11a is at so low a level as to be displayed inside the echo envelope 51 as shown in FIG. 13, and it cannot be detected in such a case. Hence, the measurement by the end peak echo method known as the most popular method for detection of surface opening flaw is disadvantageous in various aspects of the measurement as will be described below. That is, (i) When the depth d of the surface opening flaw is small, the echo from the end of the flaw is so close to the echo from any other portion of the flaw that its level and position cannot be easily distinguished from whose of the latter echo, and also the echo itself is included within an echo envelope having the corner echo as the highest value so that it cannot be detected, and at the same time since the angle probe cannot be moved over a short distance, the error of measurement is apt to be large. (ii) If the width of the flaw end is small, the level of the echo from the flaw end is small and less than the echo level at the noise level so that the echo itself cannot be detected in many cases. (iii) Since the scattering of ultrasonic waves varies depending upon the shape and size of the flaw end, the measured values are not uniform and so the accuracy of measurement is low. (iv) Even if the ultrasonic waves are transmitted after being refracted through a nominal refraction angle of the probe toward the end of a flaw, no peak echo can be obtained in some cases, and when the beam center is not coincident with the end of the flaw, the peak echo is displayed in some cases. In such case, the basis for geometrical determination of the flaw depth is lost, directly causing the reduction of the accuracy of measurement.
As one of the conventional methods for measurement of the depth of a surface opening flaw, an invention by the Applicant is disclosed in Japanese Patent Application No. Sho 60-68379. The method disclosed in the Application a vertical type probe is placed on the surface of a solid mass possibly having an opening flaw therein and also just above the flaw and the propagation time of the scattered waves derived from the ultrasonic waves incident upon the end of the opening flaw through the reflection at the flaw end is measured to determine the depth of the flaw. Also in the method in said application, it is necessary, as in the afore-mentioned reports, to detect the echo from the surface opening flaw and the position thereof. As in the reports (2) and (3), the method is limited in various respects of measurement since it is necessary to place the probe just above the surface opening flaw for measurement of the depth of the flaw.
As has been described in the foregoing, in all the conventional methods of measuring the depth of a surface opening flaw, the echo from the end of the surface opening flaw and its position are taken as indices of measurement, so that the accuracy of measurement depends upon that of detection. Thus, it is necessary to improve the above-mentioned accuracy of detection. With surface opening flaws as well as ordinary internal flaws, however, the detection of an echo from a flaw and its position is seriously influenced by the properties of the material of the object under measurement, any difference in physical phenomenon of the ultrasonic wave due to the nonuniformity of the flaw, difference in skill from one operator to another, etc. even if a transducer is used, and the flaw detector and flaw detecting parameters such as flaw detecting sensitivity, etc. are maintained as predetermined for every measurement. So, even when objects of the same kind very similar to one another in shape, dimensions, material, etc. are measured, it is difficult to measure them always with the same accuracy. So, it is rather difficult to measure objects of different kinds with the same high accuracy. Therefore, the level of an echo from the end of a surface opening flaw existent in the object and its position cannot be so uniformly detected as in the detection of a surface opening flaw artificially formed in the standard test piece or an object even if the flaw depth is the same, and the accuracy of detection is correspondingly low.